In response to the acknowledged threat that mercury poses to human health and the environment as a whole, both federal and state/provincial regulation have been implemented in the United States and Canada to permanently reduce mercury emissions, particularly from coal-fired utilities (e.g., power plants), steel mills, cement kilns, waste incinerators and boilers, industrial coal-fired boilers, and other coal combusting facilities. For example, about 40% of mercury introduced into the environment in the U.S. comes from coal-fired power plants. New coal-fired power plants will have to meet stringent new source performance standards. In addition, Canada and more than 12 states have enacted mercury control rules with targets of typically 90% control of coal-fired mercury emissions and other states are considering regulations more stringent than federal regulations. Further U.S. measures will likely require control of mercury at more stringent rates as part of new multi-pollutant regulations for all coal-fired sources.
The leading technology for mercury control from coal-fired power plants is activated carbon injection (“ACT”). ACI is the injection of powdered carbonaceous sorbents, particularly powdered activated carbon (“PAC”), upstream of either an electrostatic precipitator or a fabric filter bag house. Activated or active carbon is a porous carbonaceous material having a high adsorptive power.
Activated carbon can be highly effective in capturing oxidized (as opposed to elemental) mercury. Most enhancements to ACI have used halogens to oxidize gas-phase elemental mercury so it can be captured by the carbon surface. ACI technology has potential application to the control of mercury emissions on most coal-fired power plants, even those plants that may achieve some mercury control through control devices designed for other pollutants, such as wet or dry scrubbers for the control sulfur dioxide.
ACI is a low capital cost technology. The largest cost element is the cost of sorbents. However, ACI has inherent disadvantages that are important to some users. First, ACI is normally not effective at plants configured with hot-side electrostatic precipitators or higher temperature cold-side electrostatic precipitators, because the temperature at which the particulates are collected is higher than the temperature at which the carbon adsorbs the mercury. Second, activated carbon is less effective for plants firing high- or medium-sulfur coal, plants using selective catalytic reduction (SCR) systems to control nitrogen oxide emissions where sulfur dioxide may be converted to sulfur trioxide at the catalyst surface and plants using sulfur trioxide flue gas conditioning due to the interference of sulfur trioxide with capture of mercury on the carbon surface.
Another technique to control mercury emissions from coal-fired power plants is bromine injection with ACI. Such a mercury control system is sold by Alstom Power Inc. under the trade names Mer-Cure™ or KNX™ and by Nalco Mobotec Company under the trade name MerControl 7895™. Bromine is believed to oxidize elemental mercury and form mercuric bromide. To remove mercury effectively, bromine injection is done at high rates, typically above 100 ppmw of the coal. At 100 ppmw without ACI or other factors such as high unburned carbon from coal combustion or the presence of a flue gas desulfurization system, bromine has been reported as resulting in a change of mercury emissions of about 40% lower than the uncontrolled mercury.
Bromine, when added at high concentrations such as 100 ppmw of the coal feed, is problematic for at least two reasons. It can form HBr in the flue gas, which is highly corrosive to plant components, such as ductwork. In particular, cold surfaces in the gas path, such as air preheater internals, outlet ductwork, scrubber and stack liners, are very susceptible to corrosion attack. Also at such high injection rates, a significant amount of bromine will be emitted from the stack and into the environment. Bromine is a precursor to bromomethane, hydrobromofluorocarbons, chlorobromomethane and methyl bromide, which are known ozone depletors in the earth's upper atmosphere.